This invention relates to the art of optical micro-electromechanical systems (MEMS) devices, and more particularly, to all-optical switching using MEMS devices.
One solution for all-optical switching employs two MEMS devices each containing an array of tiltable micro mirrors, e.g., small mirrors, which can reflect light, which herein refers to any radiation in the wavelength of interest, whether or not in the visible spectrum. An optical path is established for light supplied from an input source, e.g., an optical fiber, to an output, e.g., an output fiber, by steering the light using a first micro mirror on the first optical MEMS device, the first micro mirror being associated with the input fiber, onto a second micro mirror on the second optical MEMS device which is associated with the output fiber. The second micro mirror then steers the light into the output fiber. Each fiber connected to the system is considered a port of the system, the input fibers being the input ports and the output fibers being the output ports.
A problem in the art of all-optical switching using MEMS devices is that in order to increase number of ports in the system, i.e., the number of fibers, it has been necessary to increase the number of micro mirrors employed to perform the switching function. In the prior art, as noted above, the first optical MEMS device contained all of the first micro mirrors integrated thereon and the second optical MEMS device contained all of the second micro mirrors integrated thereon. Since the size of the optical MEMS device is a direct function of the number of micro mirrors on the optical MEMS device, and the number of micro mirrors required is directly proportional to the maximum number of ports available in the all-optical switch, to increase the maximum number of ports available in the all-optical switch requires one to employ a larger optical MEMS device.
Unfortunately, limitations on manufacturing capability and the large package size have effectively limited the optical MEMS device at the present time to 1296 micro mirrors. Furthermore, even if the size of the micro mirrors could be effectively shrunk, there is still a problem in that control signals need to be brought to each micro mirror. The routing of these control signals consumes large amounts of space on the optical MEMS device, which would thus result in the optical MEMS device being very large. Additionally, there are control signals for each micro mirror that must be brought to the optical MEMS device from off of its substrate. In order to make these connections, additional large amounts of space is required on the optical MEMS device.
As a result of all these space requirements, the optical MEMS chip is quite large, and so, due to the manufacturing capability limits, the number of micro mirrors that can be placed on a single optical MEMS device is limited. The limitation on the number of micro mirrors, in turn, limits the number of ports of an all-optical switch.
Additionally, the micro mirrors presently available have a limited effective range through which they can be tilted. The limitation on the effective range further limits the number of ports that can be implemented in an all-optical switch employing such optical MEMS devices because each micro mirror on the first optical MEMS device must be able to direct the light incident on it to each of the micro mirrors on the second optical MEMS device. The ability to so direct the light is a function of the effective tilt range of the micro mirrors. In other words, greater effective tilt angle allows each micro mirror to direct its light over a greater area. For optical MEMS devices arranged as an optical switch, the greatest tilt angle required is for connections between micro mirrors in the opposing corners of the optical MEMS devices. For example, the most tilt is required by a micro mirror at the top right of the first MEMS device which must direct its light to a micro mirror at the bottom left of the second MEMS device. Thus, the size of the micro mirror array that can be employed in an optical switch is limited by the effective tilt range of its optical MEMS devices.
Increasing the separation distance between the two optical MEMS devices decreases the required tilt angle, which would allow the use of larger micro mirror arrays without changing the effective tilt range of the micro mirrors. Doing so, however, increases the beam diffraction, which, disadvantageously, requires the use of a micro mirror with a larger diameter or results in a loss of some of the light. Since using a larger micro mirror requires additional space, doing so increases the distance between the micro mirrors on the optical MEMS device, which further increases the size of the optical MEMS device for the same number of micro mirrors. As a result of increasing the size of the optical MEMS device, a greater tilt angle is required to couple the opposing corners of the opposing optical MEMS devices. Thus, essentially, additional separation of the opposing optical MEMS devices does not help to increase the number of ports due to the limited available tilt angle.
Additionally, because the package of the MEMS device is considerably larger than the area thereof that contains the micro mirrors, it is not possible with current designs to place the micro mirror areas of multiple MEMS devices directly adjacent to one another to form a single, composite, larger MEMS device. Nor does it seem likely that future designs will facilitate doing so due to the need for a large edge area on the MEMS device to make the multitude of connections that are required.
We have recognized that the limitations on the number of ports in an all-optical switch due to the constraints on the size and/or effective tilt range of the optical MEMS devices can be overcome, in accordance with the principles of the invention, by imaging one or more optical MEMS devices using an imaging system in combination with an actual other optical MEMS device, or an image thereof, to form a single virtual optical MEMS device that has the size of each of the optical MEMS devices combined. The physical size of the arrangement may be reduced by compacting the optical path, e.g., using appropriate conventional mirrors, and/or employing folded arrangements, i.e., arrangements in which there is only one MEMS device stage that does double duty for both input and output through the use of at least one conventional mirror. In one embodiment of the invention, the imaging system reproduces the angle of reflection of the light from the micro mirror. This may be achieved using a telecentric system, also known as a 4 f system.
In various embodiments of the invention, in order to combine the images and/or actual devices of multiple optical MEMS devices, the imaging systems may be at different angles to each other, and it may also be required that the lenses of the imaging system that are optically furthest from the micro mirror overlap each other. To compensate for such different angles and overlapping, in one embodiment of the invention, a prism may be inserted for each image of an optical MEMS device at the plane in which its image is formed. The prism is designed to tilt all the angles of the light for an optical MEMS device opposite to the angle between the lens of the imaging system for that optical MEMS device that is furthest from that optical MEMS device and at least the lens of the imaging system of one other optical MEMS device that is furthest from that other optical MEMS device. In another embodiment of the invention, a lens may be employed in lieu of a prism to perform the same function. In yet another embodiment of the invention, folding mirrors may be similarly employed.
The overall system is arranged to account for inversion of any images of the MEMS devices by the imaging systems employed.